Son's nose always sounds blocked

Hi

My 3 year old son's nose always sounds blocked. Ever since he was born, he's always breathed through his mouth, particularly when sleeping (and snores heavily). There are certain sounds of the alphabet that he doesn't pronounce properly, like 'M'.

He catches a cold with the seasons just like any other child; when he doesn't have a cold, his nose doesn't run and there's no pain in the nose area, and he hasn't spoken about any pain in the ear or throat area either.

We've been to the doctor, and so far she's prescribed a nasal steroid spray (Visti??). We applied that for a while as directed but I can't see any difference, though that was just as the weather was getting cooler here.

Am keen to know what the problem may be. If it's something that's physically blocking the nasal passage that needs clearing, then happy to look into that. Just don't want him to grow up with a problem that could become a bigger problem and one easier to fix now.

Thanks
Arjay